seems you have an understand of how HID keyfobs work.
then maybe you can help me? heres my story

“My company recently gave each employee a keyfob, the label on it says HID.
all I want to do is clone that exact key and then I want to write it to another key which is smaller, hopefully small enough to fit inside my nokia N78 phone, just above my sim card, there is enough space to fit two microSD (size referance) cards stacked vertically. hopefully that is enough room it fit a passive rfid chip.
I already took apart one HID keyfob and the chip inside was too big to fit”

Sorry it took so long to reply–
I’ve taken HID cards apart–but not the keyfobs. I don’t know how you can make that work, the chip itself is tiny, but the antenna that wraps around takes up alot more space. The only thought I have is wrapping your own antenna tighter but I don’t know Radio tech stuff to tell you if that has a chance of working.